1. Field of the Invention
The present invention relates to a positioning method and a positioning device for controlling the position of an actuator, and relates in particular to a positioning method and a positioning device for precisely positioning, above a track, an actuator for which bearings are employed.
2. Related Arts
Positioning devices are widely employed in a variety of apparatuses. For example, servo positioning system, for positioning a read/write head above tracks on a recording disk, are provided for a disk drive, such as magnetic disk drives, which are employed as computer storage devices. Recently, magnetic disk drives recording densities have been dramatically increased. Of the factors that have made such increases possible, one was the introduction and development of the MR (magneto-resistive) heads that are now in common use. By applying the magneto-resistive effect, MR heads have facilitated the precise detection of magnetized states, and since, as a result, higher recording face densities have therefore become feasible, as has the use of narrower recording disk track widths, they have also helped to establish a need for a precise servo system positioning function.
FIG. 8 is a diagram for explaining the prior art, and FIG. 9 is a graph showing the characteristics of ball bearings which is used to explain the prior art.
A magnetic disk drive includes a magnetic disk, a magnetic head, a spindle motor for rotating the magnetic disk, an actuator for moving the magnetic head, and a servo system, for positioning the magnetic head above a track on the magnetic disk.
The servo system can precisely position or control the head to protect against disturbances, such as the vibration of the spindle motor caused by a manufactured balance error of a ball bearing used to support the shaft of a spindle motor and a wave that the actuator receives from the disk, etc. The actuator for moving the head also includes bearings to ensure that it moves smoothly. As is shown in FIG. 8, for example, in the magnetic disk drive, an actuator 90, which is a voice coil motor (VCM) for moving a magnetic head, is rotatably fitted around a fixed shaft 91 via ball bearings 92.
It is inevitable that some slight friction will exist in the ball bearings. The types of such friction include static friction an dynamic friction. When using friction coefficients to represent friction levels, it is found that a static friction coefficient is considerably larger than a dynamic friction coefficient.
Since the track pitch is reduced for a magnetic disk, precise positioning is required when tracing a track. However, when the servo system outputs to the actuator only a minute control value (force), which is equal to or smaller than the static friction coefficient, the actuator will not move. That is, as is shown in FIG. 9, the relationship between the force F to be exerted on the bearings 92 and the rotating quantities is represented non-linearly inside the static friction region, while the relationship is represented linearly outside the static friction region.
That is, when the traveling distance is very small, bearing movement is representative of the non-linear characteristic. As a result, since bearing movement is representative of the non-linear characteristic, if a minute control value is output for the minuscule traveling distance required to follow a track, the actuator will not move in response to the output of that control value. Thus, deterioration of the track following function occurs.
Further, since the actuator will not react when a minute control value is output, it will not move unless there is a degree of increase in an error value. And as the actuator will not move until a number of errors have been accumulated, when it does move, it moves clumsily, and an overrun tends to occur. As a result, deterioration of the positioning accuracy of the servo system occurs. And since track densities have been so dramatically increased, due to the non-linear characteristic of the bearing movement, an especially outstanding deterioration in the positioning accuracy occurs.
To resolve the above problem, an actuator which does not employ ball bearings has been proposed (e.g., U.S. Pat. No. 5,355,268). In this actuator, a bearing is composed of a member having a knife-edge shape and a magnet. In such a construct, the bearing function is basically provided by point contact, which can reduce static friction.
However, the prior art has the following problems.
(1) Since the bearing function is provided by point contact, repetitive operation of a device is accompanied by wear, and proper functioning can not be provided over an extended period of time. Thus, a short service-life problem has arisen.
(2) Since the actuator is supported by the magnet, they have weak unit-shock characteristics.
It is, therefore, one objective of the present invention to provide a positioning method and a positioning device for improving the positioning accuracy attained by a servo system, even when bearings are employed which are affected by static friction.
It is another objective of the present invention to provide a positioning method and a positioning device for improving the positioning accuracy of a servo system for which special bearings are not required.
It is an additional objective of the present invention to provide a positioning method and a positioning device for the narrowing of tracks even when an actuator is used in which ball bearings are employed.
To achieve these objectives, according to a first aspect of the present invention, a positioning method for position-controlling an actuator to a target position comprises steps of:
generating a random wave signal;
detecting a positional error to the target position for the actuator;
producing a servo control signal to position the actuator to the target position from said positional error;
adding the random wave signal to the servo control signal to produce a control signal; and
driving the actuator in accordance with the control signal.
In the present invention, a random and minute vibration is constantly applied to the actuator to prevent the bearings in the actuator from assuming a static state. As a result, only dynamic friction affects the operation of the actuator, and relative to the control value, the actuator movement is representative of the linear characteristic. In this manner, positioning accuracy is enhanced.
Although the random, minute control value, which is constantly applied to the servo system, corresponds to a disturbance (noise), and as such, contributes to a loss in positioning accuracy, the improvement in the positioning accuracy produced by the linear characteristic of the bearing movement is greater than the loss referred to above. And as a result, overall positioning accuracy is improved.
Further, since the power spectrum of a random wave is flat, even when a random wave is introduced into the servo system, a exciting quantities of a resonance point of the servo system are small.
In addition, since both the position error quantities and the random wave are representative of normal distributions, the normal distributions are added together. And as the acquired sum is smaller than the result obtained by simply adding signals which are not normally distributed, the quantities of the disturbance (control quantities) can be increased.
According to one more aspect of the invention, the generating step includes a step for the generation of a random wave for so driving the actuator that a static friction state of the actuator bearings is canceled out.
As a result, the assumption of a static state by the actuator bearings can be effectively eliminated.
According to another aspect of the invention, the generating step includes a step for the generation of a random number having a predetermined cycle.
According to this aspect, since the random number is generated, a random wave can be easily produced by using a digital calculation procedure.
According to an additional aspect of the present invention, the producing step includes steps of:
Reading a signal from a track on a recording medium by a head provided for the actuator, and detecting the positional error from the read signal; and
generating the servo control signal in accordance with the positional error for following the head with the track.
According to this aspect, since the invention is employed to control a head which is following a track, control can be precisely exercised even when bearings are employed, and even when a narrower track pitch is employed.
According to a further aspect of the present invention, the producing step includes steps of:
generating a second servo control signal in accordance with the positional error when seeking a target track for the head; and
selecting the step whereat the servo control signal is generated or the step whereat the second servo control signal is generated. The positioning method further comprises a step of:
driving the actuator in accordance with the second servo control signal when the step of generating the second servo control signal is selected.
According to this aspect, since a random wave is not supplied during seek control, whereat a static state of the bearings does not occur, the admission to the servo system of an unwanted disturbance can be prevented.
According to yet one more aspect of the invention, the positioning method further comprises: a step of adjusting the level of a random wave in order to minimize a standard deviation for the position shift distance.
According to this aspect, since the level of the random wave is so adjusted that the position shift distance is minimized, deterioration of the positioning accuracy of the servo system due to a disturbance can be minimized, while the assumption of a bearing static state is avoided.
According to yet another aspect of the invention, the step whereat an adjustment occurs includes steps of:
measuring the standard deviation of the positional error; and
determining a level for the random wave which will minimize the standard deviation of the position shift distance.
According to this aspect, since the standard deviation of the position shift distance is measured automatically, the level of the random wave can be automatically adjusted to the optimal value.
According to yet an additional aspect of the present invention, the detecting step a step of:
Sampling the output of the head in accordance with a servo gate signal, and generating a positional error which indicates a distance the head has shifted away from the track,
wherein the producing step includes a step whereat the servo control signal is produced in accordance with the servo gate signal, and
wherein the generating step includes a step whereat a random wave is generated which has a frequency which is higher than the frequency of the servo gate signal.
Since the frequency of the random wave is higher than is that of the servo control signal, the actuator can be vibrated slightly without adversely affecting the accuracy of the positioning for which the servo control signal is used.